1. Field of the Invention
The present invention relates to an image forming unit for an electrophotographic image forming apparatus, such as a copying machine, a facsimile, a printer or the like, comprising an image forming mechanism including a photoconductive drum and a developing roller, a case containing the image forming mechanism so that the developer containing toner and magnetic carrier may not leak from the case during transportation, and a protective covering for the image forming unit.
2. Description of the Related Art
FIGS. 11 to 13 show a developing device of a dry developing system that supplies a developer to the developing region in the circumference of a photoconductive drum, disclosed in U.S. Pat. No. 4,676,192 and Japanese Laid-Open Patent Publication No. Hei 4-48232. This developing device has a developing device case 101 defining a developer chamber 102 containing an initial developer, i.e., a developing mixture of magnetic carrier and magnetic toner, a developing sleeve 104 for carrying the developing mixture, internally provided with a magnet roller 103, and a first toner stirring member 105.
A partition wall 106a separating the developer chamber 102 and a toner supply chamber 106 from each other is provided with a toner supply slit 107 extending parallel to the axis of the developing sleeve 104 and opening into the developer chamber 102. A second stirring member 108 for supplying the toner through the toner supply slit 107 into the developer chamber 102 is disposed in the toner supply chamber 106.
As shown in FIG. 11, a trimming blade 109 is attached to the front wall 101b of the developing device case 101 so that the gap S between the edge of the trimming blade 109 and the outer surface of the developing sleeve 104 is adjustable to form a magnetic brush of a predetermined height on the outer surface of the developing sleeve 104. In an image forming operation, the toner is applied to the circumference of the photoconductive drum 100 by the developing sleeve 104.
If the developing mixture containing the toner and the magnetic carrier leaks from the developing device case 101, the photoconductive drum 100 and a recording sheet fed to the developing device may be soiled, and the developing mixture is wasted. Therefore, as shown in FIGS. 11 to 13, semicircular recesses 110 corresponding to half the circumference of the developing sleeve 104 are formed in the opposite sealing walls 101a (only one of them is shown in FIG. 12) and strip-shaped side sealing members 111 of felt or the like are placed between the surfaces of the semicircular recesses 110 and the circumference of the developing sleeve 104, respectively, to seal the developer chamber 102.
However, since the width H0 of the side sealing members 111 in sliding contact with the opposite ends (only one of them is shown in FIG. 13) of the circumference of the developing sleeve 104 is comparatively large and the inner portions of a width H1 of the side sealing members 111 correspond to the magnet roller 103, respectively, as shown in FIG. 13, the following problems arise.
The magnetic carrier and the magnetic toner are attracted to the circumference of the developing sleeve 104 in a layer of an appropriate thickness by the magnetic force of the magnet roller 103. When the developing sleeve 104 rotates, the magnetic carrier and the magnetic toner adhering to the opposite ends of the circumference of the developing sleeve 104 rub against the side edges of the fixed side sealing members 111. Since the developer, i.e., the mixture of the magnetic carrier and the magnetic toner, is attracted by magnetic force to the circumference of the developing sleeve 104, hairs of the felt forming the side sealing members 111 are torn off and scatter, the torn hairs adhering to the developing sleeve 104 and the associated parts, thereby adversely affecting the image forming operation. During a long period of use, the side sealing members 111 are abraded by the magnetic carrier and the magnetic toner, and their sealing effect deteriorates. Consequently, the gaps at the opposite ends of the circular surface of the developer chamber 102 develop gradually with the operating time, and finally, the side sealing members 111 become unable to prevent leakage of the magnetic carrier and the magnetic toner from the developing device case 101.
Furthermore, the magnetic carrier and the magnetic toner adhering to portions of the surface of the developing sleeve 104 in sliding contact with the side sealing members 111 are pressed continuously against the surface of the developing sleeve 104, and consequently, the magnetic toner sticks to the surface of the developing sleeve 104. The magnetic toner sticking to the surface of the developing sleeve 104 may be transferred accidentally from the developing sleeve 104 to the photoconductive drum 100, forming dark stripes in the side margins of the recording sheet.
These problems are unavoidable as long as the side sealing members 111 are disposed in ranges around the opposite ends of the circumference of the developing sleeve 104, subject to the influence of magnetic fields created around the opposite ends of the magnet roller 103.